The seamless pipe or tube in general is produced by piercing and rolling a solid round billet using a piercing mill. The piercing mill includes a pusher provided along a pass line on the inlet side, a plug provided along the pass line on the outlet side, and a plurality of inclined rolls arranged opposed to one another with the plug therebetween.
A billet heated in a heating furnace is arranged on the pass line. Then, the billet has its rear end pushed by the pusher and is transported toward between the plurality of inclined rolls along the pass line. In other words, the pusher serves to transport the billet. When the billet is caught between the plurality of inclined rolls, the pusher stops its operation. The billet engaged between the plurality of inclined rolls is pierced and rolled as it helically advances, and formed into a hollow shell.
In the above-described piercing and rolling, leaf-, fin-, or lap-shaped defects (hereinafter referred to as “inner surface defects”) are generated at the inner surface of the hollow shell after the piercing and rolling because of the rotary forging effect and additional shear deformation. Therefore, measures to reduce the inner surface defects have been studied.
Methods of restraining such inner surface defects during the piercing and rolling are disclosed by JP 2000-246311 A (hereinafter as “Patent Document 1”), JP 2001-162306 A (hereinafter as “Patent Document 2”) and Japanese Patent No. 3503552 (hereinafter as Patent Document 3”). In the disclosure of these documents, the piercing and rolling may be carried out with a smaller rolling reduction than in the conventional case in order to restrain inner surface defects. If the rolling reduction is reduced during the piercing and rolling, the billet is less stably caught between the inclined rolls but such defective entry is prevented when the pusher pushes the billet from behind according to the disclosure. In short, according to these documents, the pusher is used in order to improve the defective entry of the billet that could be caused because of the reduced rolling reduction.
More specifically, as shown in FIG. 7 (that corresponds to FIG. 4 in Patent Document 1 and FIG. 4(c) in Patent Document 2), when a billet contacts inclined rolls and a plug at time t1, the roll load represented by the solid line in the figure (the load of the inclined rolls acting in the rolling direction) and the load of the plug represented by the broken line in the figure (the thrust load of the plug) increase. However, since the billet entry is unstable, the roll load and the plug load are lowered at time t1 to t3. More specifically, defective entry is caused in the period and the billet is in a slipped state. Since the billet entry is unstable, the billet is pushed from behind by the pusher at time t3. In this way, the billet is engaged between the inclined rolls, and the roll load and the plug load increase. At time t6 when the entry is stabilized, the pusher stops pushing the billet. The billet has already been engaged between the inclined rolls in a stable manner, so that the roll load and the plug load gradually increase thereafter, and at time t7 and t8 and after, the roll load and the plug load become substantially constant, in other words, the piercing and rolling reaches a steady state. According to these documents, the moving speed of the pusher is less than the speed of the billet in the rolling direction when the piercing and rolling is in the steady state. The pusher is used to improve the defective entry and it is only necessary to push the billet with the pusher when the advancing efficiency of the billet is low due to the defective entry, in other words, when the advancing speed of the billet is reduced or kept low due to the defective entry.
According to the piercing and rolling method, as shown in FIG. 8 (that corresponds to FIG. 5 in Patent Document 1), there is almost no increase in the speed of the billet in the rolling direction during the period from the start of entry to time t3 when the pusher starts to push, and the speed gradually increases after the pusher starts to push the billet at time t3. When the billet is pushed by the pusher and stably engaged, the billet departs the pusher and its speed in the rolling direction increases. After the piercing and rolling reaches a steady state, the speed in the rolling direction becomes constant.
However, if a billet is formed into a hollow shell by piercing and rolling by the disclosed methods, a greater number of inner surface defects are generated at the tip end part of the hollow shell than at the central part of the hollow shell. The tip end part with the inner surface defects may be cut off using a cutter and a reduced number of inner surface defects would remain in the seamless pipe, while the yield is lowered for the removed part. Therefore, it is preferable to reduce generation of inner surface defects at the tip end part itself rather than simply cutting off the tip end part with the inner surface defects.